This invention relates to a method of manufacturing a voltage-nonlinear resistor, and more particularly a method wherein a ZnO (zinc oxide)-based starting composition containing a small amount of metal zinc is shaped and then sintered.
In recent years semiconductor elements and semiconductor circuits such as transistors, thyristors, and ICs have been rapidly improved. Their characteristics thus improved, the semiconductor elements and circuits are used in increasing numbers in measuring devices, control devices, communication devices and power supply devices. Provided with such semiconductor elements and semiconductor circuits, the devices are successfully miniaturized and come to have a high efficiency. On the other hand, however, these devices and their parts cannot be said to be sufficiently resistant against high voltage, surge voltage and noise. It is therefore demanded that these devices or their parts be protected against an abnormally high voltage or an abnormally large noise. That is, the circuit voltage of the devices or their part should be stabilized. Voltage-nonlinear resistors meet the demand. Thus it is required that there should be developed voltage-nonlinear resistors which has an excellent voltage-nonlinearity, a large discharge capacity, a long life, and a highly resistant characteristics against an abnormally high voltage or noise.
Hitherto, to stabilize the circuit voltage of measuring devices, control devices, communication devices and power supply devices, use has been made of voltage-nonlinear resistors such as SiC varistors and Si varistors. Zener diodes have been also used for the same purpose. Recently developed is a varistor made of a ZnO-based composition containing a few additives.
The voltage-current characteristic of a varistor is generally determined by the following formula: EQU I=(V/C).sup..alpha.,
where V is the voltage across the varistor, I the current flowing through the varistor, C a constant, and .alpha. a nonlinearity coefficient. When .alpha.=1, the varistor is an ordinary resistor covered by the Ohm's law. The larger is .alpha., the better voltage-nonlinearity. Generally, the varistor characteristic is determined by C and .alpha.. Here, the varistor characteristic is expressed by .alpha. and a starting voltage V.sub.1 mA at 1 mA.
Known SiC varistors are made by sintering SiC particles bonded together by a ceramic binder. The voltage-nonlinearity of the SiC varistors is determined by the dependancy of the contact resistance of SiC particles on the voltage applied to them. Thus, the value of C can be controlled by changing the thickness of the varistor, measured in the direction in which current flows through the varistor. But the nonlinearity coefficient of SiC varistors is relatively small, usually 3 to 7. Further, to manufacture an SiC varistor it is necessary to sinter an SiC mass in a non-oxidizing atmosphere.
Si varistors, whose voltage-nonlinearity owes to p-n junctions formed in an Si mass. The value of C cannot therefore be controlled over a broad range. Similarly, the voltage-nonlinearity of Zener diodes owes to p-n junctions formed in them. Despite their very good voltage-nonlinearity, Zener diodes cannot make resistor elements for a high voltage. They are disadvantageous also in that they are not sufficiently resistant against a surge voltage.
Other known voltage-nonlinear resistors are ceramic varistors made of a ZnO-based composition containing bismuth oxide, cobalt oxide, manganese oxide, antimony oxide and the like. These are rather varistors of new type. They exhibit an excellent voltage-nonlinearity which owes to the sintered masses of ZnO-based composition themselves. But the rate at which their V.sub.1 mA varies in positive direction when a large impulse current flows through them much differs from the rate at which their V.sub.1 mA varies in negative direction when a large impulse current flows through them. That is, ceramic varistors do not exhibit a symmetrical voltage-current characteristic. Thus they are not sufficiently stable and therefore not sufficiently reliable.
Other ceramic varistors are known which are made of a ZnO-based composition containing nickel oxide, barium oxide and the like or a ZnO-based composition containing rare earth element and cobalt oxide, but not containing bismuth oxide. Indeed these ceramic varistors exhibit a voltage-current characteristic less asymmetrical than that of the ceramic varistors made of a ZnO-based composition containing bismuth oxide among other additives. Further, their V.sub.1 mA varies but very little. But they are less resistant against a surge voltage. In addition, they do not function for a sufficiently long time.